This invention relates to a printing machine and particularly to a device for determining an amount of ink in a multicolor offset printing machine.
In a multicolor offset printing, four color printing is normally carried out. In the case of four color printing, four printing plates for four colors, that is, yellow, magenta, cyan and black are mounted on four printing units, respectively. Each printing unit has an inking arrangement for adjusting an amount of ink to be supplied.
The inking arrangement has an ink fountain in which a blade assembly is provided along the outer surface of an ink fountain roller. The amount of ink to be supplied is adjusted on the basis of the space between the tip of the blade assembly and the outer surface of the ink fountain roller and on the basis of a rotational angle or speed of the ink fountain roller. The blade assembly is divided into a plurality of blade pieces, each blade piece of which is adjusted independently.
The inking arrangement is controlled by a device for determining an amount of ink to be supplied (hereinafter referred to as a determining device). Into the determining device is inputted data of rates of pattern areas which show distribution of patterns in a predetermined area of a printing plate. The rates of pattern areas are measured with respect to each of four printing plates and are memorized on a medium such as a magnetic card or in a microcomputer.
The determining device has a converting unit for converting a rate of pattern area with respect to each blade piece into a value of the space between the tip of the blade piece and the outer surface of the ink fountain roller on a basis of a predetermined conversion function. On the basis of each value of rates of pattern areas, each blade piece is so adjusted as to move the tip of the blade piece with respect to the circumferential surface of the ink fountain roller. When the space (blade space) between the tip of the blade piece and the outer surface of the ink fountain roller is not suitable, an operator adjusts each blade piece through a key board of a correction input unit. In this manner, an amount of ink supplied from the inking arrangement is adjusted with respect to each printing unit.
However, in the above conventional determining device, there are defects that the tip of each blade piece and the circumferential surface of the ink fountain roller wear gradually with the passage of time thereby to generate a deviation between an initially determined blade space and an actual blade space, and a quality of printing is decreased. The deviation can be eliminated in such a manner that a conversion function is slightly amended or modified to determine a blade space again. However, this determination is not performed with respect to each blade piece of a blade assembly but performed with respect to as a whole with respect to each blade assembly corresponding to each printing unit. Further, the blade assembly and the outer surface of the ink fountain roller do not necessarily wear uniformly through their all regions in its longitudinal direction but may wear partially. In case that there is a large partial wear, a corresponding blade piece is replaced by a new one. In this case, each blade space has to be readjusted to keep the balance between the blade space of a replaced blade piece and the blade spaces of other old blade pieces. This readjustment is performed with respect to each blade piece, and, however, there is a delicate difference in characteristic between one blade piece and other blade pieces. Therefore, the blade spaces of all blade pieces cannot be recovered to their most suitable condition easily and the readjustment need much time and works.
In this manner, in the conventional determining device, as a conversion function is determined as a whole with respect to each printing unit, the determining device cannot cope with a change with the passage of time generated with respect to each blade piece.
Further, in the conventional printing unit, in a case that an amount of ink to be supplied with respect to each printing unit is once determined, the rotational angle and speed of an ink fountain roller is kept at its initially determined value and an adjustment to cope with a wear of the ink fountain roller is not carried out. Accordingly, a precise readjustment of each blade piece must be carried out on the basis of printing data in the past in order to increase quality of printing. In a conventional printing press, whenever the patterns of printing, the kind of color and the order of the printing are changed, the readjustment of rotational angle or speed of the ink fountain roller is needed according to the kind of ink to be supplied because each ink has a different characteristic in viscosity, color or the like.